In the manufacture of glass, glassmaking materials are melted in a glassmelting furnace by heat provided from burners which combust fuel with oxygen. The fuel can be combusted with air as the source of the oxygen, or with a stream containing a higher oxygen content than that of air. The furnace must be manufactured of material that can withstand the very high temperatures that prevail within the furnace. The materials of construction often employed, which typically include silicas and related materials, are well known.
However, the conditions within the glassmelting furnace have been known to cause corrosion of the inner surfaces of the furnace, especially of the roof (“crown”) over the glassmaking materials. The most widely used material for the crown is silica brick for soda-lime-silicate glass furnaces. Alkali vapors (mostly NaOH and KOH) generated from the glass batch material in the glassmelting furnace react with silica refractory brick and faun over time a glassy silicate material on the inner surface of the crown. When a sufficient concentration of alkali oxides (mainly Na2O and K2O) accumulates in the glassy silicate layer, the glassy material can become fluid enough to drip directly into the molten glass in the furnace or to run along the silica refractory surface and over other refractory surfaces in the furnace and dissolve or dislodge some of the refractory particles which fall into the molten glass. This corrosion is undesirable as it leads to a loss of material in the crown, which eventually leads to expensive repairs or replacement of the crown, and because the corrosion products have been known to fall into the glassmaking materials in the furnace and to cause defects in the glass product.
U.S. Pat. Nos. 3,238,030 and 3,240,581, teaches that the formation of the silicate layer on the crown surface is to be encouraged, evidently in the belief that the layer prevented other corrosion products from causing defects in the glass product.
U.S. Pat. No. 3,238,030 teaches that alkali compounds such as sodium sulfate that form in the glassmelting process should be permitted to reach the material on the crown surface, whereupon the sulfate is subjected to conditions that convert the sulfate to more silicate on the material on the crown surface. However, for reasons mentioned above, that silicate layer eventually becomes a source of defects in the glass product and a sign of corrosive damage to the crown.
U.S. Pat. No. 3,240,581 teaches the introduction into the glass furnace atmosphere of additional amounts of alkali (sodium) even beyond the amounts that are already in the glass furnace atmosphere from the glassmaking materials. Here, too, is a teaching that is now understood to lead to conditions at the crown surface that cause defects in the glass product and damage to the crown.